Method and apparatus for grinding small objects



Nov. 5, 1946. FRUTH 2,410,491

METHOD AND APPARATUS FOR GRINDING SMALL OBJECTS Filed Jan. 6, 1944 4 Sheets-Sheet l INVENTOR.

Nov. 5, 1946; H. F. FRUTH METHOD AND APPARATUS FOR GRINDING SMALL OBJECTS 4 Sheets-Sheet 2 Filed Jan. 6,' 1944 \ARERQT INVENTOR.

NOV. 5, 1946. FRUTH 2,410,491

METHOD AND APPARATUS FOR GRINDING SMALI] QB JECTS I Filed Jan. 6, 1944 4 Sheets-Sheet s INVENTOR.

fi mz H. F. FRUTH 2,410,491 METHOD AND APPARATUS FOR GRINDING SMALL OBJECTS Nov. 5, 1946.

Filed Jan. 6, 1944 4 Sheets-Sheet 4 Q VINVENTOR. HM ffrufla latented Nov. 1946 lib-UTE. 'SAES T OFICE METHOD AND APPARATUS FOR GRINDING SMALL OBJECTS Application January 6, 1944, Serial No. 517,175

7 Claims. 1

The present invention relates to methods and apparatus for grinding small objects, and more particularly, to improved methods and apparatus for finish grinding the edges of piezoelectric crystals, in order to remove irregularities therefrom, to increase the activity thereof, and to produce polished edge surfaces thereon. This application is a continuation in part of co-pending application Serial No. 492,203, filed June 25, 1943, Hal F. Fruth, now Patent No. 2,387,136 issued October 16, 1945.

In the manufacture of certain articles or parts which have relatively sharp edges formed at the intersections between the surfaces of each part or article it is frequently desirable, if not essential, that the edges of the article be ground for the purpose of removing irregularities therefrom. Thus, in the manufacture of piezoelectric crystals adapted for use in communication circuits and more particularly, for use in crystal microphones, radio transmitting and receiving systems and the like, the crystal blanks are cut from the crystal stock, are ground to the dimensions required to provide the desired frequency characteristics, and are then edge ground to remove irregularities from the edges thereof. According to conventional practice, all finish grinding operations are performed by hand, and are effected by bringing the desired surface or edge to bear against an abrasive surface and manually moving the bearing surface of the crystal across the abrasive surface. These finished grinding operations are performed in two steps, i. e. face grinding and edge grinding, either of which may be performed ahead of the other. Both operations as practiced by conventional manual methods, are tediously slow, and require skilled labor in the performance thereof. Also, in the finished grinding of crystals for use in ultra-high frequency circuits, having a thickness of approximately '7 mills or less, the waste due to breakage during the finish grinding of .the

crystals by hand is unduly high. Moreover, the

finished crystals obtained by such grinding methods are not entirely satisfactory in operation. The operating difiiculties which have been experienced are attributable in large part to the fact that it is apparently impossible with conventional edge grinding methods to obtain crystals which are free of edge irregularities. Under certain temperature conditions, those edge irregularities or discontinuities which are present in a finished crystal tend to produce spurious vibrating frequencies which are difiicult to stop when once started. Moreover, the minute points which remain at the edges of the crystal have a tendencyto break away, causing crystal dust which remains on the surfaces of the crystals. The dust particles will sometimes change the frequency of vibration of the crystal or cause the vibration of the crystal to be entirely arrested, which of course, is undesirable from an operating standpoint. Another principal objective in the edge grinding of the crystals, is that of producing peak crystal activity. With conventional manual methods of edge grinding, activity peaking of a crystal is difficult to obtain, with the result that for a given number of crystal blanks, the percentage of waste due to insufficient activity of the finished crystals is excessively high.

It is an object of the present invention therefore to provide improved methods and apparatus for finish grinding the edges of small objects or articles to remove edge discontinuities therefrom.

It is another object of the invention to provide improved methods and apparatus for grinding piezoelectric crystals in an improved manner such that the disadvantages of the manual grinding methods now commonly in use are obviated, and an improved crystal structure having more stable operating characteristics. and acceptacle activity is obtained.

According to another object ofthe invention, a controlled method of crystal grinding is provided, which may be utilized on a variable time basis with a minimum expenditure of manual labor, toedge grind small objects so that they conform with precision accuracy to a predetermined standard. 7

In accordance with a further object of the I invention, the objects are edge ground in batches and in a manner such that face surface abrasion thereof is substantially eliminated.

It is a still further object of the invention to provide an improved method and apparatus for edge grinding piezoelectric crystals in batches at high speeds and with a minimum expenditure of labor.

According to a further object of the invention, the edge grinding of a batch of crystals is accomplished Without any abrasion or grinding of the crystal faces.

It is still another object of the invention to provide "an improved method and improved apparatus for grinding a small object, in which the grinding time is reduced by centrifugally actuating the object into engagement with a relatively movable abrasive surface, thereby to pro- 3 duce a contact pressure between the object and the surface which exceeds that producible by gravity pull upon the object.

According to yet another and more specific object of the invention, the grinding of the object is accomplished by alternately employing gravity force and centrifugal force to determine the contact pressure between the abrasive surface and the contacting surface portions of the object.

It is a further object of the invention to provide an improved method of edge grinding a batch of piezo-electric crystals to impart substantially peak activity to an exceedingly high percentage of the crystals in the batch.

In accordance with yet another object of the invention, the crystal activity peaking is obtained by repeatedly subjecting decreasing portions of the crystals of a batch to random edge grind operations for predetermined time intervals which respectively correspond to expected activity peaking of certain crystals Within the batch, and by removing the acceptably peaked crystals from the batch at the end of each grinding operation.

The invention, both as to its organization and method of operation, together with further objects and advantages thereof, will best be understood by reference to the specification taken in connection with the accompanying drawings, in which:

Fig. 1 is a side view of improved edge grinding apparatus which is characterized by the features of the present invention and may be utilized to practice the improved method of grinding crystal blanks or the like;

Fig, 2 is an end sectional view of the apparatus shown in Fig. 1;

Fig. 3 is an end view of a portion of the apparatus shown in Fig. 1;

Fig. 4- is a circuit diagram illustrating the arrangement of the circuit for energizing the driving motor of the apparatus shown in Fig. 1;

Fig. 5 is a fragmentary sectional View illustrating the disposition of crystals within one of the edge grinding tubes embodied in the apparatus shown in Fig. 1;

Fig. 6 is an end sectional View of the grinding tube shown in Fig. 5;

Fig. 7 is a characteristic crystal activity curve indicating the activity of a typical crystal plotted as a function of edge grinding time;

Fig. 8 is a fragmentary sectional view illustrating the manner in which the crystal faces are coated in order to prevent face abrasion thereof during the edge grinding operations;

Fig. 9 is a side view illustrating a modified arrangement of the edge grinding apparatus;

Fig. 1c is an end view partially in section illustrating the movable parts of the apparatus shown in Fig. 9;

Fig. 11 is a view schematically illustrating the driving arrangement for the apparatus shown in Figs. 9 and and Fig. 12 is a side view illustrating means for tilting the apparatus shown in Figs 9 and 10.

Referring now to the drawings, and more particularly to Figs. 1, 2, 3 and 4 thereof, the improved edge grinding apparatus there illustrated comprises a base It having two upstanding bracket pieces I2 and I3 rigidly mounted thereon at spaced apart points therealong. These bracket pieces are respectively provided with bearings I B and it which rotatably support a shaft It upon which a plurality of edge grinding tubes II are mounted. More specifically, the tubes II, which are preferably made of glass, are arranged in the form of a cone having a long axis coinciding with the axis of rotation of the shaft I6. As thu arranged, each tube may be considered as being tilted with respect to the axis of rotation of the shaft 58. Preferably, each tube is inclined at an angle of approximately twenty degrees relative to the shaft I8. For the purpose of holding the tubes II upon the shaft IS in the described positions therefor, a supporting structure is provided which comprises a small disc I4 mounted adjacent the bracket l2 for rotation with the shaft, and a large disc I5 which is disposed adjacent the opposite bracket !3. The latter disc is provided with a hub l-5a which may be utilized in set-screw mounting the disc upon the shaft is for rotation therewith.

As best shown in Fig. 1 of the drawings, a nut 23 is used to clamp the disc I5 against the flanged end 22a of a sleeve 22 which is splined or key connected to the shaft IS, the nut 23 being threaded onto, the threaded outer surface of this sleeve in order to clamp the disc I4 against the sleeve flange 22a. With this arrangement, the disc I4, as viewed in Fig. 1 of the drawings, may be moved to the left away from the disc I 5 for the purpose of removing the tubes I! from between the two discs hi and I 5. The opposite ends of each grinding tube I I are closed by means of rubber stoppers 26a and 2Ia which are both utilized in rigidly supporting the associated tube upon the two discs I4 and I5. More in detail, the support is completed by respectively providing the stoppers Mia and 2m with bent metal stems and 2| which are adapted to be received within oppositely disposed holes provided at the periphcries. of the discs L5 and I4 respectively. Thus the upper one of the grinding tubes I I as shown in Fig. 1 of the drawings, is provided at its right end with a stopper 20a having fixedly secured thereto a stem 28' which enters an opening provided' in the side of the disc I5 at the upper peripheral portion thereof. At its left end, this tube is closed by means of a stopper a having fixedly securedthereto a stem ZI which enters an opening in the upper peripheral portion of the disc Id. The other grinding tubes I I of the apparatus are likewise supported between different peripheral portions of the two discs I4 and I5. With this arrangement, the tubes may be readily detached from the supporting discs by moving the disc [4' to the left in the above-explained manner, thereby to permit withdrawal of the stems 2!. individual to the various tubes from the openings provided at the periphery of this disc.

A variable speed driving motor 30, which is mounted upon the base I0, is provided for imparting rotary movement tothe shaft I6 and the grinding tubes I I supported thereon. This motor is equipped with a double ended rotor shaft 29, one end of which carries a pulley 28 which is connected by a driving belt 2? to drive a second pulley 26 rigidly mounted upon the left end of the shaft I6. The opposite end of the rotor shaft 29 is mechanically connected to drive a speed reducing gear box 3! having a take-01f shaft 32 which mounts a crank disc 33. This disc is pinconnected by means of a pin 35 to one end of crank arm 34 which is utilized to pivot a rheostat wiper arm 36 back and forth between two extreme positions. More specifically, the arm 36 is pivotally supported by means of an axis pin 37 and is pivotally connected to the actuating end of the crank arm 34 by means of an aXis pin 38. At its opposite end, the arm 36 carries a wiper 39 which is arranged to be moved back and forth over a resistor element 49 in response to pivotal movement of the arm 36 between the extreme angular positions thereof.

The described rheostat facilities are utilized to vary the speed of operation of the driving motor 39 back and forth over a predetermined speed range. To this end and as best shown in Fig. 4: of the drawings, the motor 36 is arranged to be energized from a source of current indicated by the bracketed terminals 52 over a circuit which includes the in-circuited portion of the resistor 49, the wiper 39, and the contacts of a manually operable on and off switch M.

In utilizing the above described apparatus for the purpose of grinding the edges of piezoelectric crystals, the crystals are disposed within the tubes H. To produce random changes in the position of each crystal relative to the tube within which it is confined, a baffle or obstruction 43 is provided at one or more points along each tube. Each bafile may comprise a small strip of adhesive tape extending longitudinally of the tube in the manner shown in Figs. 5 and 6 of the drawings. Depending upon the type of random movement of the crystals which is desired, the bafl'le strips may be positioned at different points intermediate the ends of each tube H, or may be provided only at the ends of each tube.

In the operation of the apparatus shown in Figs. 1, 2, 3, 4, 5 and 6 of the drawings, each of the tubes H is loaded with a charge of crystals and abrasive material, in the manner more fully explainedbelow, after which the tubes are stoppered, and are mounted between the two discs l4 and IS. The switch 41 may now be operated for the purpose of energizing the driving motor 39. When operation of the driving motor is thus initiated, each tube H, in its tilted position relative to the axis of rotation of the shaft I6, is rotated about this axis to define the surface of a cone. In the normal use of the apparatus, the base I0 is supported upon a horizontal surface so that the shaft lt occupies a substantially horizontal position. Accordingly, as each tube II is rotated about this shaft, it is alternately tilted in opposite directions with respect to the horizontal, whereby the crystals disposed therewithin tend to slide back and forth longitudinally of the tube. The maximum tendency of the crystals to slide to the left within a confining tube ll obviously occurs when this tube is rotated to a position directly above the shaft l6. Conversely, the maximum tendency for a tube confined batch of crystals to move from the left end of the tube toward the right end thereof occurs when the tube occupies a position directly below the shaft l6. Intermediate the extreme positions indicated, each tube is obviously tilted at a lesser angle with respect to the horizontal, with the result that the defined tendency of the crystals to slide in one direction or the other longitudinally of the tubes is lessened. Under the conditions now under consideration, this sliding action of the crystals is alone produced by gravity pull upon the crystals. Gravity pull upon each crystal also causes relative circumferential movement between the inner surface of the supporting tube and the crystal. Thus each tube is effectively rotated through one revolution about its own longitudinal axis during each revolution of the tube about the shaft [6. Further, gravity pull upon each crystal tends to hold the crystal against the bottom portion of its confining tube, with the result that so long as the tube is rotated at slow speeds, the crystal is slid around the-entire inner circumference of the tube during each revolution of the tube about the shaft it. Thus it will .be understood that during rotation of the tubes H, the batch of crystals supported within each tube are slid back and forth within the tube and at the same time are moved around the inner surface of the tube.

As indicated above, the purpose of the motor controlled rhecstatic facilities comprising the resistor t9 and the wiper 39 is that of varying the speed of operation of the motor 39 over a wide speed range. To this end, the wiper 39 is driven back and forth across the resistor 39 under the influence of the driving force exerted thereon through the gear box 3f, the crank disc 33, and the crank arm 3 As this Wiper is moved in a direction for increasing the resistance of the circuit for energizing the motor 39, the speed of rotation of this motor decreases to correspondingly decrease the speed of rotation of the grinding tubes II about the shaft i6. Conversely, as the wiper 39 is moved in the opposite direction to decrease the resistance of the motor energizing circuit, the speed of rotation of the motor 30 is increased to produce a corresponding increase in the speed of rotation of the tubes H about the shaft l6. Preferably, the permissible resistance change of the resistor 49 due to the movement of the wiper 39 thereacross is such that the speed of rotation of the tubes H about the shaft I6 is varied back and forth between an upper limit of 500R. P. M. and a lower limit of 25 R. P. M. In this regard it will be noted that as the motor 3% speeds up, movement of the wiper 39 across the resistor 49 is accelerated to increase the rate of motor speed up. Conversely, as the motor speed is decreased from the upper limit of the speed range, movement of the wiper 39 across the resistor G9 is decelerated to decrease the rate at which the motor slows down. As a result, any selected period of high speed rotation of the tubes ll about the shaft i6 is relatively short when compared with the remainin period of slow speed rotation of the tubes about the shaft.

As the speed of rotation of each tube ll about the shaft 56 is increased, the increased centrifugal force acting upon the crystals confined therewithin tends with increasing effectiveness to move the crystals to the right ends of the tubes as viewed in Fig. l of the drawings, and lessens the tendency of the crystals in the tube to move to the left when the tube is rotatedto a position directly above the shaft l6. As a result, the zone of longitudinal movement of the crystals within each tube is decreasingly narrowed and is increasingly shifted toward the right end of the tube as the speed of rotation of the tube about the shaft i6 is increased. At a predetermined critical'speed located somewhere within the defined limits of the speed range, the centrifugal force acting upon the crystals within each tube becomes sufficient to throw the crystals radially outward from the shaft it against the outermost portion of the inner surface of the tube. As the speed of rotation of the tubes H is increased above this critical value, the increasing centrifugal force acting upon each crystal serves in-' creasingly to enhance the contact pressure between the crystal and the abrasively coated inner surfaces of its supporting tubes at the points of engagement therebetween. This increase in contact pressure between each crystal and the engaging tube surface, which pressure may exceed;

7 by ten or fifteen times the pressure produci'ble by gravity pull upon the crystal, is obtained withoutv eliminating relative sliding movement between each crystal and the engaging surface of the tube. Such. relative sliding movement is, however, confined to movement of the crystals longitudinally of their confining tubes, since once the crystals are thrown outwardly against the inner surfaces of the tubes, the tendency for the tubes to rotate around the crystals is eliminated. Sliding movement of the crystals longitudinally of the tubes is controlled by the changing effect of gravity'pull upon the crystals. Thus as each tube is moved. from a substantially horizontal position to a position beneath theshaft i and then back to a horizontal position, the centrifugal and gravity forces add or combine to increase the contact pressure and to increase the tendency to slide each crystal within the tube towards the right end of the tube. On the other hand, as each tube is moved from a substantially horizontal position to a position above the shaft I6 and then back to a horizontal position, the net force tending to more each crystal within the tube towards the right end of the tube is represented by the difference between the centrifugal force acting upon the crystal and the gravity force acting upon the crystal. Accordingly each crystal may be held back somewhat in its movement toward the upwardly tilting right end of its supporting tube. The net result is that each crystal is slid longitudinally of its confining tube during each revolution of the tube about the shaft [6. Finally, the crystals are all stacked against each other at the right ends of their respective confining tubes.

As the speed of rotation of the tubes ll about the shaft I6 is decreased, the crystals are held at the right ends of their respective supporting tubes under the influence of centrifugal force until such time as the speed of rotation of the tubes is lowered to the critical value at which the gravity pull upon the crystals exceeds the centrifugal force acting upon the crystals. Below this critical speed, the crystals are only held in contact with the inner surfaces of the tubes by gravity pull with the result that sliding movement of the crystals longitudinally and circumferentially of the tubes is resumed.

From the foregoing explanation it will be understood that at tube speeds below the critical value at which the centrifugal forces acting upon the crystals exceed gravity pull upon the crystals, the contact pressure with which the crystals are held in engagement with the inner surfaces of their confining tubes is determined solely by gravity pull upon the crystals. At tube speeds in excess of the critical value, the centrifugal forces acting upon the crystals determine the contact pressures between the crystals and their supporting surfaces, and may exceed by several times the contact pressures producible by gravity pull alone. When the latter condition prevails, the abrading action is much more rapid, due to the greater contact pressures between the crystals and their abrading surfaces. It will also be understood that after the critical speed of tube rotation is exceeded, only a short time interval is required to move all of the crystals to the right ends of their respective supporting tubes, and that once the crystals are stacked against each other at the rightends of the tubes no further abrading action occurs. It is for this reason that the described rheostatic control facilities for the motor 30 are deliberately arranged to shorten 8 the. periods of high speed rotation of the grinding tubes. Thus by so doing, effective abrading of the crystals is obtained during the predominant portion of each complete speed cycle period.

In the modified arrangement of the apparatus shown in Figs. 9, l0, l1 and 12 of the drawings, the grinding tubes 55 are journalled within bearlugs 51 and 58 which are supported within oppositely disposed openings arranged around the outer peripheries of two spaced apart supporting discs 53 and 54. These discs are set screw mounted by means of set screws 53a and 54a upon a rotatable shaft 55 which is journalled within bearing members to; and 52a carried by upstanding bracket pieces 5| and 52, respectively. The identified bracket pieces are rigidly mounted upon a base member 5t. Stoppers 53 are used to close the ends of the grinding tubes. Collars 60 and El, set screw mounted upon each tube 56 upon opposite sides of the disc 5d, are utilized to prevent end play of the tubes 56 relative to their supporting discs 53 and 54. For the purpose of rotating the shaft 55 thereby to rotate the tubes 56 about this shaft, a driving motor 61 is provided which i mounted upon the right end of the base member 50 in the manner shown in Fig. 12 of the drawings. This motor is beltconnected to drive the shaft 55 by means of the pulleys l2 and 65 and the belt 66. Rotation of the tubes 56 relative to the stationary parts of the apparatus is also utilized to rotate these tubes relative to their supporting discs 53 and 54. To this end, each tube, a viewed in Fig. 9 of the drawings, carries an enlarged driving pulley 62 at the right end thereof. These pulleys are set screw mounted, or otherwise fixedly secured to their respective supporting tubes, and are provided with belt grooves which are in alignment transversely of the structure and are also aligned with a belt groove 5% cut around the bearing tube 5264 within which the shaft 55 is journalled. A belt 54 which is disposed within the groove 52b and passes around the outer peripheral portions of the several pulleys 52 in the manner shown in Fig. 10 of the drawings, is utilized to rotate the pulleys and the tubes 55 relative to the supporting discs 53 and 54 in response to rotation of the named parts about the shaft 55.

In order to insure random grinding of crystals disposed within the grinding tubes 56, the base 58 is tiltably supported by means of an axis rod '54 and spaced apart bearing brackets l5 upon a second base member 16. A slow speed motor 11, mounted upon the base member 16, is provided for tilting the base 55 and the parts carried thereby back and forth about the axis rod 14, whereby the grinding tubes, 56 are alternately tilted in different directions relative to the horizontal. More specifically, the rotor shaft of the motor i? is provided with a crank disc 18 which is crank connected by means of a crank 19 and the axis pins and 8! to the right end of the base 50.

As best shown in Fig. 11 of the drawings, the motor i7 is arranged to be energized from a current source indicated by the bracketed termirials 13 over a circuit which includes the contacts of an on-off switch H. The motor 61 is connected for energization from this source over a circuit which also includes the contacts of the switch H and the contacts of a communicating device. This device comprises a rotary conductive disc 65, an insulating segment 69 which provides a non-conductive segment at the disc periphery, and a brush HI Wipingly engaging the 9 periphery of the disc. The disc may be actut d at a slow speed by any suitable driving means so that the operating circuit for the motor 6? is intermittently opened in the manner described below.

In considering the operation of the apparatus shown in Figs. 9, 10 and 11 of the drawings it will be understood that to load the tubes 55 with charges of crystals and abrasive material, the apparatus is tilted about the rod is, the stoppers 63 are removed fromthe tubes at the uppermost ends thereof, and measured charges of crystals and abrasive material are then introduced into the various tubes. After the tubes are thus loaded, the stoppers 63 may be re-inserted in the open ends of the tubes, following which the apparatus is fully conditioned to perform the desired grinding operation. To initiate this operation, the switch H is operated to its closed circuit position to energize the motor ll and to complete a circuit through the commutating disc 55 and the engaging wiper 78 for energizing the driving motor iii. In operating, this motor serves to rotate the tubes 55 about the shaft 55 in an obvious manner. The motor ll concurrently operates to tilt the apparatus back and forth about the aXis rod 14. Coincident with the rotation of the tubes 56 about the shaft 55, the belt 64 in its engagement with the belt glove 52b of the stationary bearing hub 52a functions to rotate each of the tubes relative to the supporting discs 53 and 3 at a reduced speed which is determined by the relative diameters of the pulleys 52 and the bearing hub 52a.

In this embodiment of the apparatus, the motor 6? is operated at a speed which will insure that the crystals confined within each grinding tube 53 are thrown radially outward from the shaft 55 under the influence of centrifugal force to engage the ever changing portion of the inner surface of the tube which is farthest removed from the axis of rotation of the shaft 55. Preferably this speed is sufficiently high to provide a contact pressure between the inner surface of each grinding tube and the contacting surfaces of the crystals confined therewithin, exceeding by ten or fifteen times the contact pressure which is alone producible by gravity pull upon the crystals. This high contact pressure insures rapid grinding, and the centrifugal force producing the same serves to prevent the crystals from being rotated with the tubes around the longitudinal axes of rotation of the tubes. Relative sliding movement required to produce the desiredabrading action is brought about by the rotation of each grinding tube 55 relative to the supporting discs 53 and 56. Thus, as the crystals within each tube are held by centrifugal force against the surfaces of the confining tube farthest removed from the shaft 55, the rotation of the tube relative to the two supporting discs serves to move the inner surface of the tube around the crystals, and centrifugal force acts in the manner explained above to prevent rotation of the crystals with this surface. Preferably the grinding tubes 56 are rotated relative to the supporting discs 53 and 54 at only a fraction of the speed of rotation of the tubes about the shaft 55, thereby to prevent too rapid abrasion of the crystals.

As will be evident from the above explanation, once the crystals are centrifugally actuated into engagement with the inner surfaces of their confining tubes, the contacting surfaces of each crystal will not ordinarily be changed until the centrifugal actuating force is removed. It is for zontal.

10 this reason that the commutating device comprising the time actuated commutating disc 68 and the wiping brush 10 is provided. Thus, when this disc is rotated to a position wherein the brush engages the insulating segment 6%, the circuit for energizing the driving motor t'l is obviously interrupted. As a result, the rotary movement of the grinding tubes 56 relative to the supporting discs and about the shaft is decelerated to a standstill. When the speed of rotation of the tubes 56 about the shaft 55 is reduced to a predetermined value, to correspondingly decrease the centrifugal forces acting upon the crystals confined within the tubes, these forces are exceeded by gravitational forces acting upon the crystals, with the result that the crystals fall down to engage the bottom surfaces of the respective tubes within whichthey are confined. As the crystals within each tube fall, they are obviously tumbled about, with the result that the positions thereof within the tube are changed at random. After a predetermined time interval required for the insulating segment 59 to traverse the wiper 15, the operating circuit for the driving motor 67 is re-completed, thereby to reinitiate rotation of the tubes about the shaft 55 and relative to the two supporting discs. As the speed of rotation of the tubes about the shaft 55 is accelerated to the normal value, the crystals within each tube are again thrown outwardly under the influence of the centrifugal forces acting thereon. Incident to the movement of the crystals which occurs within the tubes during the tube decelerating and accelerating periods, the positions ofthe crystals are changed at random, thereby substantially to insure that new surface portions of each crystal will be brought into contact with the inner surface of the tube within which the crystal isconfined. It will thus be understood that the commutating devicecom prising the disc 58 and the wiper iii, in its opera tion periodically to de-energize the driving motor direction and then the other relative to the hori--- As a result, the crystals within each I grinding tube 56 will obviously slide longitudinally of the tube both when centrifugally actuated to engage the inner surface of the tube and when gravity actuated to engagethis surface. During the decelerating and accelerating periods the sliding movement of the'crystals longitudinally of the tubes 55 may be utilized to change at random the positions of the crystals within the tubes. To this end, bafile strips 43 of the character described above and shown in Figs. 5 and 6 of the drawings may be provided in each grinding tube at one or more points along the inner surface thereof to produce the desired crystal position change.

The present improved method of edge grinding, as practiced by utilizing either embodiment of the improved apparatus described above, may conveniently be considered in its application to the finish grinding of the edges of piezoelectric quartz crystals.

As previously indicated, such crystalsare initially sliced from the mother crystal, following which they are diced to the approximate desired dimensions and are then rough ground more nearly to conform to the desired rectangle dimensions on a grinding wheel. After the rough grinding operations are completed, two finish grinding operations are required, namely, the grinding of the crystal faces to the desired thickness for the purpose of obtaining the desired resonant frequency characteristics, and the edge grinding of the crystals for the purposes of ii.- creasing the activity thereof and of removing edge discontinuities therefrom. These two finish grinding operations may be performed in any desired order, but both are necessary in order to obtain a satisfactory crystal having satisfactory operating characteristics. In the usual crystal structure, the crystal length is somewhat greater than the crystal width. These dimensions are taken into account in determining the inner diameters of the tubes ll or 56 which are utilized in the two embodiments of the above disclosed apparatus. More specifically, the purpose of the edge grinding operation is that of producing a rounded chamfer or bevel at the junction point between each pair of intersecting surfaces of each crystal. Accordingly, the diameter of each tube H or 56 is properly chosen to produce th desired average chamfer radius at the edges defining theintersections between the crystal surfaces. Thus to provide a relatively broad chamfer it is necessary that the tube diameters rather closely approximatethe longest dimension of each crystal. On the other hand, to produce a. sharp ohamfer the tube diameter should be relatively large as compared with the longest dimension of each crystal. Preferably the tube diameters as related to the dimensions of the crystals are about as shown in Fig. 6 of the drawings. As there illustrated, the diameter of the illustrated grinding tube l I or 56 is so chosen that when a crystal is supported at its edges with its long axis extending longitudinally of the tube, a second crystal may be supported thereabove with the long axis thereof extending transversely of the tube and with the adjacent surfaces of the two superimposed crystals in spaced apart relationship. With this arrangement, the upper crystal is prevented from changing the contact pressure between the inner surface of the tube and the contacting edges of the crystal, and the presence of the lower crystal beneath the upper crystal is prevented from reducing the contact pressure between the inner surface of the tube and the engaging edges of the upper crystal. Moreover, the described relative diminishing of the tube diameters and the length and width of the crystals minimizes face abrasion of the crystals as the edge grinding operation proceeds. In this regard it is noted that if the finish grinding of the crystal faces to impart a predetermined frequency character thereto is completed before the edge grinding operation .is started, further crystal face grinding should, if possible, be entirely eliminated during the edge grinding operation in order positively to prevent the resonant frequencies of the crystals from being changed. In order to minimize such crystal face abrasion during the edge grinding operation the faces of each crystal may be coated or otherwise covered before the edge grinding operation is started. This coating may take the form illustrated in Fig. 8 of the drawings, wherein the faces of a crystal fragment I? are shown as being coated with thin layers Fig and I lb of a protective material. Preferably a Water soluble glue is employed as the coating material, this particular product having the desired advantages of being easily applicable to the crystal faces and of being easily removable therefrom after the edge grinding operation is completed. Thus in order to remove the glue from the crystal faces it is only necessary to dip the crystals in boiling water after the edge grinding operation is completed. It is noted, however, that adhesively coated paper strips, scotch tape or the like, may be applied to the crystal faces to perform the protective function with equal success. The provision of such a coating or covering positively precludes face grinding of the crystals during the edge grinding operation. In addition, the coating applied to the faces of each crystal increases the mass of the crystal, thereby to increase the contact pressure between the edges of the crystal and the abrasive grinding surface and thus enhance the abrading action. If extremely fast grinding is desired, each crystal face covering may take the form of a small metal wafer or plate of lesser length and width than the crystal, which is adhesively secured to the central portion of the crystal face by a water soluble glue. Such face coverings produce a marked increase in the mass of each crystal, with an accompanying increase in the speed of edge abrasion.

As indicated above, to condition either embodiment of the disclosed apparatus for a grinding operation the tubes l I or 56 are each loaded with a batch of crystals and a charge of abrasive material. The abrasive material may comprise a small amount of diamond powder or boron carbide, the particular grinding speed being determined to a large extent by the size of the grinding particles employed. Thus, if rapid grinding is desired diamond powder of No. 100' to 150 screen mesh may be used. On the other hand, if less rapid grinding and a higher polish of the ohamfer surfaces is to be obtained, No. 320 to No. 400 screen mesh grinding material may be utilized. The amount of grinding material used in each tube will, of course, depend upon the dimensions of the tube. For example, when tubes having a length of about 60 inches and an internal diameter of 1.0625 inches are employed, satisfactory grinding is obtained if approximately two carats of No. 1G0 to 150 screen mesh diamond powder is used in each tube. On the other hand, if the finer grinding powder of from N0. 320 to No. 400 screen mesh is employed, approximately two to three carats of the grinding material should be used in each tube.

After each grinding tube is loaded with a batch of crystals and a charge of abrasive material in the manner just explained, operation of the driving means of either embodiment of the apparatus may be started for the purpose of initiating the edge grinding operation. The abrading action which occurs when the first embodiment of the disclosed apparatus is used may accurately be considered as being divided into two parts, i. e. that which is produced when gravity pull alone determines the contact pressure between the edges of each crystal and the abrasively coated inner surface of its confining tube, and that which occurs when the crystals are centrifugally actuated to bring the edges thereof to bear against the abrasively coated inner surfaces of the grinding tubes. During those intervals when gravity pull upon the crystals predominates, i. e. the periods when the grinding tubes H are being rotated at slow speed, each crystal within each tube is slid back and forth longitudinally of the tube over the inner abrasive coated surface thereof. Concurrently therewith and due to the rotation of the tubes about the shaft IS, the inner surface of each grinding tube is slid around the contacting edge surfaces of each crystal which it houses. As this sliding movement proceeds, the crystals tend to align themselves along the length of their respective enclosing tubes with only the straight edge sections thereof bearing against the inner surfaces of the enclosing tubes. The abrasive material is of course soon spread over the inner surface of each tube to produce an abrasive surface against which the edge surfaces of the crystals bear. Due to the abrasive action of this material, the high points of the crystals are cut away as the crystals are slid relative to the surfaces of their enclosing tubes. In this connection it is pointed out that the movement of the crystal edges longitudinally of the tubes prevents transverse scratches from being produced in the chamfered surfaces which are soon formed at the edges of each crystal. Similarly, the sliding of thecrystals circumferentially of the abrasive surfaces against which they bear prevents the chamfered surfaces from being scratched longitudinally, incident to the sliding of the crystals back and forth within their enclosing tubes.

Preferably the angle of inclination of each tube l I with respect to the horizontal shaft H3 is such that when the tubes are being rotated about this shaft at slow speed, a crystal disposed at the elevated end of a tube will just slide to the other end of the tube before the angle of inclination of the tube is reversed. More specifically, the lengths of the tubes are preferably such that when the angle of inclination is approximately 20 degrees, the desired full length sliding movement of a crystal longitudinally of its confining tube is obtained. The longitudinal movement of the crystals along the tubes during the periods of slow speed rotation of the tubes about the shaft it is also utilized to change the positions of the crystals within the tubes so that different edge sections thereof are brought to bear against the abrasively coated inner surfaces of the tubes. To consider one aspect of the crystal tumbling movement, it may be assumed that the uppermost crystal as shown in Fig. 6 of the drawings is slid into a position where it is transversely aligned with the baffle strip 43. As the illustrated tube l l in which this crystal is disposed is rotated through one revolution about the shaft it, one edge of the strip 63 engages one side of the crystal so that during continued rotation of the tube the crystal is'turned over to reverse the edges thereof which bear against the abrasively coated inner surface of the tube. The crystal is not only turned over so that alternately faces thereof face downwardly, but in addition is turned end for end so that all eight edges of the crystal are at one time or another brought to ,bear against the abrasively coated inner surface of the tube. Incident to the turning of the crystals end for end the corner edges thereof are also subjected to the action of the abrasive material so that they too become smoothly rounded. The particular set of oppositely disposed edges of the particular crystal under consideration which remain in contact with the abrasively coated surface of the tube during the longitudinal sliding movement of the crystal through the tube depends upon the particular position at which the crystal is brought to rest after it is moved out of transverse alignment with the baflle strip 43.

From the above explanation it wil1 be understood that the edge grinding of the crystals disposed within the tubes H is, during slow speed rotation of these tubes about the shaft l6, produced with a completely random action; that is, the back and forth movement of the crystals with respect to the abrasive surfaces do not follow any predetermined or ascertainable pattern, but differs for each crystal. When, however, the speed of rotation of the tubes ll about the shaft I6 is increased to a point such that the crystals are centrifugally actuated to engage the abrasive surfaces relative sliding movement of the crystal edges circumferentially of the tubes is stopped in the manner previously explained, the relative sliding movement being confined exclusively to movement of the crystals longitudinally of the tubes within which they are confined. It is during this portion of each tube speed cycle that the high contact pressure and resulting rapid abrasion of the crystal edges are obtained. During each such period, moreover, the particular set of edges of each crystal which bear against the inner surface of the enclosing tube remain unchanged. As between successive high speed periods, however, the random movement of the crystals within the tubes suflices to produce the desired change in the position of each crystal necessary to bring a different set of edges into contact with the abrading surface during the sucseeding high contact pressure g g period- On an average basis, therefore, all eight edges of each crystal are in a given time interval chamfered or ground to almost precisely the same extent.

When the embodiment of the apparatus disclosed in Figs. 9, 10 and 11 of the drawings is utilized to perform the edge grinding operation, the action is substantially similar to that just described with reference to the first disclosed embodiment of the apparatus. In the second embodiment of the apparatus, however, the transverse grinding of the crystal edges is produced both during the periods of low contact pressure grinding and the periods of high contact pressure grinding. Thus, since the tubes 55 are at all times positively rotated relative to their supporting discs 53 and 5d, relative movement of the crystal edges oircumferentially of the inner grinding surfaces of the tubes is produced so long as the motor Bl is operating. This insures rapid abrasion of the crystal edges and also provides for the cutting of these edges along the two paths of abrasion, i. e. transversely and longitudinally of the crystal edges, so long as the abrading action is being carried on. Otherwise, the abrading action obtained with the second disclosed embodiment of the apparatus is exactly the same as described above.

As indicated above, the edge grinding of the crystals is not only for the purpose of removing discontinuities of the crystal edges to enhance the stability of operation thereof, but is also for the purpose of increasing the activity of each crystal to an acceptable value. It has been found that when the activity of a crystal is plotted as a function of edge grinding time, a sawtooth characteristic curve having a number of peaks and valleys is obtained. A typical curve of this character, plotted from experimental data, is'indicated at A in Fig. 7 of the drawings. From aconsideration of this curve it will be noted that the activity of the particular crystal in question peaks at the points B, C and D, respectivelycorresponding to 4, l2 and 28 hours of edge grinding time, utilizing random edge grinding wherein the contact pressures are alone determined by gravity pull upon the crystal. Between these peak points valleys representing non-acceptable crystal activity. On an empirical basis it has also been found that for a given batch of crystals, approximately 25 per cent of the batch are possessed of substantially matching activity-edge grinding time peaks representing acceptable crystal activity after initial predetermined edge grinding interval. Further, it has been found that roughly 25 per cent of theremaining non-acceptable crystals of the batch have substantially matching activity peaks at a second point farther along the time axis of their characteristic curves. This activity peak matching of the remaining non-acceptable crystals of the batch proceeds on about the percentage basis as the characteristic curves of the individual crystals and are further compared on a continued edge grinding time basis.

In practicing the pr sent improved methods of crystal edge grinding to obtain finished crystals having acceptable activity, successively smaller portions of the crystals of a given batch are repeatedly subjected to edge grinding operations of the character described above for predetermined time intervals. More specifically, the entire batch of crystals is initially edge ground for a period ranging from four to six hours, following which the crystals are segregated from the abrasive ma terial, cleaned and tested for activity. Those crystals, approximating 25 per cent of the batch. having acceptable activity are separated from the batch and are not subjected to further edge grinding. Th remaining non-acceptable crystals of the batch are subjected to a further edge grinding operation for an additional period of from four to six hours, following which the activity testing step is repeated for the purpose of again picking out the crystals which have been edge ground to possess acceptable activity. This process of repeatedly edge grinding the non-accept able crystals on a timed basis and of separating the acceptable crystals from the batch at the of each grinding operation, is continued until such time as the predominant portion of the crystals have been edge ground to possess the desired activity. It has been found that by extending this process over a sufficiently long period of time, approximately 95 per cent of the crystals in a given batch may be successfully ground to possess the desired activity.

While different embodiments of the invention have been disclosed, it will be understood that various modifications may be made therein which are within the true spirit and scope of the inven tion.

I claim:

1. The method of grinding an object which comprises supporting said object upon the abrasively coated inner surface of an elongated tubular member which is tilted relative to an axis of rotation, rotating said tubular member about said axis to slide said object back and forth longitudinally of said tubular member, and varying the speed of rotation of said tubular member back and forth between a lower limit at which said object is freely slidable longitudinally of said member and an upper limit at which said object is centrifugally restrained at'the end of said tube furthest removed radially from said axis of ro tation.

2. In the manufacture of piezoelectric crystals each having a number ofactivity peaks which are separated along an edge grinding time axis,

16 the method of edge grinding the crystals of a given batch to produce substantially peak activity for the major portion of the crystals in said batch, which comprises concurrently supporting each of the crystals of the batch along a pair of opposed edges upon abrasive material within a tube, rotating the tube at a speed so that the abrasive material moves relative to the crystals thereon, and in such manner subjecting all of the crystals of said batch to an edge grinding operation for a selected time interval corresponding to expected activity peaking of at least a portion of said crystals, separating the acceptably peaked crystals from the batch, subjecting successively smaller portions of the remaining crystals to repeated corresponding edge grinding operations for selected time intervals respectively corresponding to expected activity peaking of said successively smaller portions of said remaining crystals, and removing the acceptably peaked crystals from the remainder between successive ones of the repeated such grinding operations in the tube. V

3. The method of grinding the edges of an object which comprises supporting said object along its opposite edges upon the abrasively coated inner surface of an elongated tubular member, centrifugally actuating said object so that its edges are held in pressure engagement with said surface, and moving said member to abrade the edges of said object transversely thereof and to move the ends of said member alternately above and below the center of said member, thereby to slide said object back and forth longitudinally of said member and thus abrade the edges of said object longitudinally thereof.

4. The method of grinding the edges of an object which comprises, supporting said object along its opposite edges upon the abrasively coated inner surface of an elongated member, centrifugally actuating said object so that its edges are forced into pressure engagement with said surface, moving said member to abrade the edges of said object transversely thereof, intermittently decreasing the centrifugal force acting upon said object, and gravitationally actuating said object to shift the position thereof relative to said surface each time the centrifugal force acting upon said object is decreased, whereby different opposed edges of said object are brought into contact with said surface.

5. The method of grinding an object which comprises, supporting said object upon the abrasively coated inner surface of an elongated tubular member which is tilted relative to an axis of rotation, rotating said tubular member about said axis to slide said object back and forth longitudinally of said tubular member, varying the speed of rotation of said tubular member back and forth between a lower limit at which said object is freely slidable longitudinally of said member and an upper limit at which said object is centrifugally restrained at the end of said tube furthest removed from said axis, and contacting said object with a point fixed relative to said surface during said relative sliding movement, thereby to change at random the edges of said object which bear against said surface.

6. The method of grinding the edges of an object which comprises, supporting said object along its opposite edges upon the abrasively coated inner surface of an elongated tubular member, centrifugally actuating'said object so that its edges are held in pressure engagement with said surface, moving said member to abradev the edges of said object transversely thereof and to move the ends of said member alternately above and below the center of said member, thereby to slide said object back and forth longitudinally of said member and thus abrade the edges of said object longitudinally thereof, and intermittently changing the edges of said object which are brought to bear against said surface by said centrifugal actuation of said object.

7. The method of grinding the edges of an object which comprises, supporting said object along its opposite edges upon the abrasively coated inner surface of an elongated tubular member, centrifugally actuating said object so that its edges are held in pressure engagement with said surface, moving said member to abrade 18 the edges of said object transversely thereof and to move the ends of said member alternately above and below the center of said member, thereby to slide said object back and forth longitudinally of said member and thus abrade the edges of said object longitudinally thereof, intermittently decreasing the centrifugal force acting to produce pressure engagement between the edges of said object and said surface, and gravitationally shifting the position of said object relative to said surface during at least a portion of the intervals when said centrifugal force is decreased, thereby to change the edges of said object which are brought to bear against said surface by said centrifugal actuation of said object.

HAL F. FRUTH. 

